We have shown that the uropod-bearing lymphocyte present in human peripheral blood may be separated into two distinct populations. The first (approximately 22 percent) is not affected by the presence of serum or plasma. The second (approximately 78 percent) does not form uropods in the presence of serum. The serum inhibitory factor is a large molecular weight protein that is associated with the low density lipoproteins. Both subpopulations of uropod-bearing lymphocytes appear to be thymic derived cells. Most of these cells appear not to have Fc receptors, complement receptors, or easily detectable surface immunoglobulins. Work in our laboratory is presently concerned with the development of a suitable immunoassay to accurately quantitate the concentration of the serum inhibitory factor in whole human serum and plasma. We have already applied the finding that the uropod-bearing lymphocyte is a large subpopulation of T cells to certain disease states. Using the uropod as a non-contrived, functional marker of this subpopulation of lymphocytes, we have quantitated the cells in x-linked hypogrammaglobulinemic patients and in patients with chronic lymphocytic leukemia. We are presently studying patients with other types of lymphomas, leukemias, and patients with lipid disorders.